Des que vam veure aquell llampec al juliol, la meva mare tanca sempre la finestra quan comença a ploure.

Des que means since or ever since when it is followed by a whole clause.

• Des que vam veure aquell llampec... = Since we saw that lightning flash...

By contrast, des de is usually used before a noun or time expression:

• des de juliol = since July
• des de casa = from home

So in this sentence, des que is used because it introduces the clause vam veure aquell llampec.

Vam veure is the normal Catalan past tense for we saw.

It is the periphrastic past, which is very common in spoken and written Catalan:

• vam
  • infinitive
• vam veure = we saw

This tense expresses a completed action in the past. Here, it refers to the specific moment when we saw that lightning flash.

Yes, historically it comes from anar (to go), but in this construction it functions as part of the past tense, not with its literal meaning of go.

So:

• vam veure does not mean we go to see
• it simply means we saw

This is one of the most important Catalan verb patterns to learn.

Aquell means that.

Catalan distinguishes demonstratives more clearly than English:

• aquest = this
• aquell = that

So aquell llampec means that lightning flash / that bolt of lightning, referring to something more distant in time, space, or shared memory.

Here it sounds like the speaker is referring back to a memorable event: that lightning flash in July.

Llampec usually means flash of lightning or lightning bolt.

It is masculine singular:

• el llampec
• aquell llampec

A related word is llamp, which can also mean lightning or bolt of lightning. In everyday usage, both may appear, but llampec often emphasizes the flash.

Because al is the contraction of a + el.

So:

• al juliol = literally at the July
• naturally in English: in July

Catalan often uses a with months in this kind of expression. Depending on context, al juliol can mean:

• in July
• that July

Here it refers to the July when that event happened.

Catalan very often uses the definite article before a possessive with family members:

• la meva mare = my mother
• el meu pare = my father

This is normal Catalan, even though English does not use the in this situation.

So la meva mare is not unusually emphatic; it is just standard usage.

Because the sentence describes a habit that started after that past event and still continues now.

• Des que vam veure... sets the starting point in the past.
• la meva mare tanca sempre... describes what she does regularly now, ever since that event.

So the present tense is very natural here. It means something like:

• Ever since that happened, my mother always closes the window...

Tanca is the 3rd person singular present of tancar (to close).

So:

• jo tanco = I close
• tu tanques = you close
• ell/ella tanca = he/she closes

Here it agrees with la meva mare, so tanca means she closes.

Yes, adverb placement in Catalan is somewhat flexible.

In this sentence:

• la meva mare tanca sempre la finestra

means my mother always closes the window

You could also hear:

• la meva mare sempre tanca la finestra

Both are possible. The original version is perfectly natural. The position of sempre can slightly affect emphasis, but not the basic meaning.

Catalan normally uses the definite article with specific countable nouns in cases like this.

So:

• tanca la finestra = closes the window

That is simply the normal way to refer to a particular, contextually understood window.

Literally, it means:

• when it begins to rain

Word by word:

• quan = when
• comença = begins / starts
• a ploure = to rain

So the whole clause means when it starts to rain.

Because començar is normally followed by a + infinitive in Catalan.

So:

• començar a ploure = to start raining
• començar a parlar = to start speaking
• començar a córrer = to start running

That a is required in standard Catalan.

It is singular because ploure is an impersonal weather verb.

English says it starts to rain, but that it does not refer to anything real. Catalan usually does not use an equivalent dummy subject.

So:

• comença a ploure = literally begins to rain
• natural English: it starts to rain

The verb stays in the 3rd person singular, which is normal for weather expressions.

Yes, but the meaning would be slightly different.

• quan plou = when it rains / when it is raining
• quan comença a ploure = when it starts to rain

The sentence with comença a ploure focuses on the moment the rain begins, which fits the idea that the mother closes the window as soon as the rain starts.

Yes, it is standard and helpful.

The first part:

• Des que vam veure aquell llampec al juliol

is a subordinate time clause. The comma separates it from the main clause:

• la meva mare tanca sempre la finestra quan comença a ploure

This makes the sentence easier to read and is the normal punctuation choice.

Because they refer to different time frames:

• vam veure = one completed event in the past
• comença = a repeated present situation

The sentence combines:

1. a past cause or starting point
2. a present habit that happens whenever rain begins

That mix of tenses is completely natural in Catalan.